Until late last century, though, it was more or less limited to what could be picked up by human eavesdroppers of various kinds. In one form, it consisted of suave young diplomatic attaches hanging around exotic capitals attending elegant high-level cocktail parties and picking up snatches of gossip to pass on for their spymasters to make what use of them they could.

Sometimes, if they were lucky, these emissaries could suborn a secretary or a cleaner in a prime ministerial or presidential office and garner some prime morsel of information.

Everyone did it, although how useful intelligence gathered by such means was has long been debated. In any event, human intelligence has long since been overtaken by the electronic kind. Now, everyone with the money employs enormous resources, with numberless satellites and vast computer farms, to pluck intelligence from the ether.

This has been the case for at least 30 years and while the public may not have paid much attention, it has not been a huge secret. The largest is the multi-billion dollar United States National Security Agency, although China and Russia almost certainly match it. Britain has its own extensive apparatus at its Government Communications HQ and even New Zealand, fathoms down the scale, has the Government Communications Security Bureau.

The immensity of the electronic information gathering has, however, recently been brought to more public attention by the revelations of the rogue contractor with the NSA, Edward Snowden, who, from a bolthole in Russia, has been drip-feeding information about seemingly indiscriminate NSA snooping around the world.

His revelations have been condemned by Western intelligence officials and politicians as deeply damaging to their efforts to protect national security. That, of course, was to be expected, but it is chillingly plausible and may well be true. True or not, there is little doubt that Snowden's leaks have also raised concerns about just how pervasive the intrusion on electronic communications has become.

The revelation this week that the NSA has for 10 years been eavesdropping on the personal cellphone of the German chancellor, Angela Merkel, has brought a new burst of outrage. To an extent, the indignation is manufactured. As the US director of national intelligence, James Clapper, told Congress yesterday, such spying is a basic pillar of intelligence operations and has gone on for decades. The Germans would have been well aware of that.

But the uneasy feeling that tapping into the communications of allies crosses some kind of threshold is unmistakable. Some countries avoid the possiblity with arrangements like the Five Eyes pact under which the US, Britain, Canada, Australia and New Zealand agree to share intelligence with the understanding they will not spy on one another. Others, including Germany, are now seeking similar agreements. They might help although, in all reality, it is unlikely to eliminate such spying altogether.